1. Field of the Invention
The present invention relates to a small zoom lens, and particularly to a zoom lens suitable for a lens shutter type compact camera with less limit of backfocus.
2. Description of the Prior Art
As a zoom lens comprising a first lens unit having a negative focal length, a second lens unit having a positive focal length and a third lens unit having a negative focal length in order from an object side, a zoom lens of a wide angle system has been heretofore known from Japanese Patent Application Laid-Open Publication No. 75108/19833, Japanese Patent Publication No. 50327/1983 and the like. However, these zoom lenses are intended to be used as photographic lenses for single-lens reflex, and have a long backfocus. Therefore, even at the shortest focal length end in which projection of a lens from a camera is shortest, the distance from the extreme end of a lens to an imaging plane is long. When this is employed for a photographic lens for a lens shutter type compact camera, the lens becomes too long so that it can not be employed for a camera desired to have a compact property when being carried.
On the other hand, as zoom lenses comprising three lens units similar to that as described above in which the backfocus at the shortest focal length end is short and which are suitable for a lens shutter type compact camera, those disclosed in Japanese Patent Application Laid-Open Publication Nos. 271214/1988, 72114/1989 and the like have been known. In these zoom lenses, overall length of the lens is compact despite high variable power. However, a moving amount of the third lens unit is large in order to realize the high variable power.
In these zoom lenses comprising three lens units, three units are required to be moved independently of one another. When the unit is moved by use of a cam in a conventional manner, three different cam grooves have to be made for a single cam cylinder so that a torque necessary for driving the cam cylinder increases. In order to relieve the torque, it is necessary to make an inclination of each cam small, and the diameter of the cam cylinder becomes large. In addition, the length in a direction of an optical axis of a member for holding and moving the lens unit also becomes long. Particularly in the case where the moving amount of the third lens unit is largest among three lens units, this moving amount greatly influences on the compactness in a direction of an optical axis.
The zoom lenses proposed in the aforementioned Japanese Patent Application Laid-Open Publication Nos. 271214/19988, 72114/1989 and the like are suitable for a lens shutter type compact camera which is reduced in the overall length of the lens despite the high variable power. However, in these zoom lenses, the curvature of field in the periphery of an image plate has a very large value, and a change caused by the variable power is also large. For this reason, an image quality of a peripheral portion of an image plane in an infinite far depiction and a photography of a plane object lowers, and with respect to a solid object at a finite distance, a depiction having a sense of incompatibility results.
On the other hand, recently, it has been desired that the zoom lens be provided with a close photographic function. If an attempt is made to obtain a high variable power and compact zoom lens, a refracting power of each of lens units constituting a zoom lens tends to be large. In a method for focussing by independently moving lens units, which is a system ordinarily employed, such as a focussing system for moving a first lens unit, a rear focus system and the like, a change in aberration caused by focussing is so large that the aberration becomes difficult to be well corrected in the range of distance of an object from an infinite far point to a near point. In the system for extending the first lens unit, it is necessary to make a diameter of a front lent larger in order to secure a quantity of marginal light when the first lens unit is extended, impairing the compactness. On the other hand, in the rear focus system, the moving amount of rear lens units for focussing is fifferent even with respect to the object at the same distance according to the focal length, and a focussing mechanism tends to become too complicated.